1. Field of the Invention
The present invention relates to the structure of a vehicle door frame in which hemming (empty hemming) is formed at an edge of a metal sheet and also relates to a method of producing such a door frame.
2. Description of Related Art
In vehicle door frames (door sashes), regardless of press-molded products or roll-formed products, sometimes hemming (empty hemming/press working), in which an edge of a single metal sheet is folded back, is performed instead of seaming, in which two metal (iron-based or aluminum-alloyed) sheets are joined. Typically, hemming is performed on the edge (vertical edge) of a design portion of a vertical pillar sash on the vehicle exterior side thereof. Principal purposes of hemming are to improve the outward appearance of the edge of a metal sheet and prevent this edge from rusting, and conventional hemming consists of an essential three-step process: down-flanging (90-degree bending), prehemming and rope-hemming. The first bending step (down-flanging) bends the edge of a metal sheet to substantially 90 degrees. The second bending step (prehemming) further bends the edge by approximately an additional 45 degrees (i.e., approximately 135 degrees with respect to the metal sheet before the first bending step), and the final step (rope-hemming) folds the edge back (by approximately 180 degrees with respect to the metal sheet before the first bending step) onto the major part of the metal sheet (the remainder of the metal sheet) while leaving an arc-shaped cross sectional portion (space/clearance/air pocket) in the folded edge. Rope-hemming is sometimes replaced by flat-hemming (0-degree (180-degree) bending) that folds the edge so as to completely lie on the major part of the metal sheet (the remainder of the metal sheet). A sealer (waterproofing agent) is usually applied to the door frame edge on which hemming has been performed to prevent the edge from rusting. In this specification, “hemming” (empty hemming) is defined as press-working an edge of a single metal sheet by folding the edge back without sandwiching any other metal sheet therebetween, whereas “seaming” is defined as press-working to join a first metal sheet with a second metal sheet by sandwiching the second metal sheet between the first metal sheet and the folded-back edge of the first metal sheet.
Conventional hemming consists of an essential three-step process: down-flanging, prehemming and rope-hemming (or flat-hemming) as described above, which has been common technical knowledge in the related art.